This invention relates to a method and apparatus for cooling electronic components.
Modern electronic devices, such as computers, consist of electronic components mounted on circuit boards. When these electronic devices operate, they generate heat. This heat must be removed from the devices or they may malfunction or may even be damaged or destroyed.
Most electronic components have a range of temperature at which they are designed to operate, hereinafter referred to as "normal operating range". Devices operating in the normal operating range operate according to product specifications with regard to speed, time to failure, and the like. Most electronic components have a maximum operating temperature. Electronic components operating at a temperature above the normal operating range, but below the maximum operating temperature, perform the desired function, but operate in a degraded fashion; for example, they may operate more slowly, they may be less tolerant of voltage variations, they may be less tolerant of electrical "noise", or they may fail after a shorter than normal period of time.
One technique for removing heat is to cause a gas, such as air, to move across the surface of the electronic components, or across a heat sink which contacts the electronic component. However, due to their low densities, gases are limited in the amount of heat that they can remove from electronic components. Therefore, as the electronic components, such as integrated circuits, incorporate more and more circuits, and operate faster and faster, they may generate enough heat such that circulating gas is not sufficient to remove the heat generated by the electronic components.
An alternate method for removing heat is to cause a liquid to contact a heat sink which contacts the electronic component. Because of their greater densities, moving liquids are capable of removing much greater amounts of heat than gases, but they involve either immersing the entire circuit board in the cooling liquid, or plumbing systems to circulate the liquid to the electronic components, or both. These plumbing systems often contain mechanical elements such as pumps, which are prone to failure. The designer of a cooling system for electronic components must therefore choose between the greater cooling capacity of a liquid cooling system and the lower cost and simplicity of a gas or air cooling system. Immersing the entire circuit board in the cooling liquid limits the choice of cooling liquid, because the cooling liquid must be non-corrosive to the circuit board and all its components and non-conductive. In addition care must be taken so that the cooling liquid does not get contaminated.
A part of most modern gas and liquid cooling systems for electronic devices is a heat sink. A heat sink is a thermally conductive material, such as aluminum, in physical contact with at least one electronic component which generates the heat. The heat sink transmits the heat to a cooling liquid or gas, or to some heat transport mechanism such as a cold plate.
A heat sink is normally physically attached to the electronic component or components by some means such as a thermally conductive adhesive, prior to its attachment to the circuit board. It covers approximately the same surface area of the circuit board (often referred to as the "footprint") as that of the electronic component or components.
If an electronic component produces a large amount of heat relative to its surface area, it may be necessary to use a so-called "extended surface" heat sink. Some extended surface heat sinks have a surface area and footprint that is larger than the surface area and footprint of the electronic component or components with which it is in contact. This is disadvantageous, however, as the extended surface heat sink may take up space on a circuit board that the designer could use for other purposes. Alternatively, extended surface heat sinks with compact footprints are taller than normal heat sinks. This is disadvantageous because it affects how closely circuit boards can be positioned.
There are typically several heat sinks on a circuit board. Heat sinks are normally designed specifically for either liquid or gas cooling. A heat sink designed for gas cooling maximizes the surface area of the heat sink that is exposed to the cooling gas, within the physical constraint of the design of the electronic device. A heat sink designed for liquid cooling must include a method of containing the liquid.
Another part of some gas or liquid cooling systems for electronic devices is a cold plate. A cold plate is a thermally conductive material such as copper or aluminum. A cold plate may directly contact the electronic component or, more commonly, may contact a heat sink which is in direct contact with the component; however, it is not normally attached to the electronic component or the heat sink prior to the component being placed on the circuit board. A cold plate is typically much larger in at least one dimension than the electronic components, and cover a much larger surface area than any of the electronic components. As ordinarily used, there are rarely more than one or two cold plates on a circuit board.
A factor which complicates the designer's decision whether to use a liquid, air or gas cooling system is that different types of components generate different amounts of heat. For example, devices containing computer CPU (Central Processing Unit) logic integrated circuits typically produce much greater amounts of heat than components such as computer memory chips. Another example of a component that produces greater amounts of heat than other components is a power amplifier in a microwave communications or radar system. This may result in a so-called "hot spot" on the circuit board. Most circulating gas systems or gas convection systems provide cooling that is relatively uniform over the surface of the circuit board. They are generally not well adapted to concentrate cooling gas at a "hot spot".
An example of a system that is adapted to cooling "hot spots" is illustrated in Bell U.S. Pat. No. 4,498,118, issued Feb. 5, 1985. In Bell, a cooling module is slidably mounted in opposed guide rails between two circuit boards. In the cooling module are orifices, through which a pressurized cooling fluid is directed towards the components of the adjacent circuit boards. A system such as Bell would require that the pattern of orifices be specially designed for the pattern of components on each circuit board, which makes production of the cooling module relatively complicated and expensive. Redesign of the circuit board, or replacement of either circuit board with another circuit board would require redesign and replacement of the cooling module. The cooling module would take up space in the rack of a cabinet that might otherwise be occupied by another circuit board. Only one cooling fluid is utilized, and the fluid circulates across the surface of the heat sink.
Other examples of systems that are adapted to cooling "hot spots" are illustrated in two application for a U.S. Patent by Novotny, Ser. No. 367,355 and Ser. No. 367,369, co-pending with this application and assigned to the same assingee as is the present application. In the Novotny inventions, a fluid is directed to the components through expansion valves. Only one cooling fluid is utilized, and the fluid circulates across the surface of the heat sink.
Liquid cooling systems are well adapted to cooling "hot spots" if the cooling liquid can be circulated directly to the hot spot. However the liquid cooling system has a greater capacity than is necessary to cool the devices that produce lesser quantities of heat. This can result in an unnecessarily complicated and expensive cooling system.
Designers of electronic devices try to make the devices as "fail safe" as possible. This is done by minimizing the consequences of failure of a component. One technique is to avoid situations in which the failure of one component results in the failure of the entire device. Often failure of a component or set of components of a cooling system can cause the entire device to fail. For example if the device is a liquid cooled computer, and the pump in the liquid cooling systems fails, the components may rapidly overheat, which may cause an unplanned computer shutdown. A frequently used technique for minimizing the consequences of the failure of a component of the cooling system is to include redundant components. For example, in gas cooling systems, it is common practice to employ multiple fans to force the cooling gas across the electronic components.
Prevention of a sudden failure is of particular interest in the computer art. If a computer experiences an unplanned shutdown, users may experience great inconvenience and expense. If a computer fails suddenly, valuable information can be irretrievably lost. However, if a potential failure can be detected and delayed a more graceful, planned shutdown can be performed. If the computer is a part of a computer network, the operations being performed by the failing computer may even be able to be transferred to another computer.
A cooling system may have two different methods of cooling the electronic components. For example, Altoz U.S. Pat. No. 4,635,709, entitled "Dual Mode Heat Exchanger" and issued Jan. 13, 1987 has a cooling system which circulates ambient air over the electronic components in an aircraft pod. If the aircraft travels at sufficiently high speed, the so-called "ram effect" makes the surrounding air too warm for effective cooling. For that situation, Altoz provides a second cooling method which uses evaporative cooling. In the evaporative cooling method, heat is transferred to a liquid which evaporates and is exhausted through a hydrophobic filter membrane. Systems such as Altoz which utilize both liquid and gas cooling techniques are often referred to as "hybrid" cooling systems.
A second example of a hybrid cooling system is described in Parmerlee et al. U.S. Pat. No. 4,315,300, entitled "Cooling Arrangement for Plug-in Module Assembly" and issued Feb. 9, 1982. In Parmerlee, et al., each heat generating module has a metallic heat plate which facilitates the transfer of heat from the electronic components to the parallel metallic sides which support the modules. Each parallel side is provided with fluid passageways for carrying a cooling liquid for removing heat transferred from the modules by way of the heat plate.
A key objective in Parmerlee et al. is to ensure that each module receives cool air at the same temperature of the air cooling the other modules. Thus, the liquid in Parmerlee is primarily concerned with providing cooling air of a temperature that is uniform from one module to another, but is not primarily concerned with cooling electronic components directly.
While Parmerlee et al. may achieve the objective of providing each module with cool air at the same temperature of the cooling air of the other modules, it is not as effective at cooling components directly. The amount of heat a cold plate can conduct is directly related to the cross sectional area of the cold plate. If it is necessary to increase the amount of heat conducted by the cold plate, the designer must make the cold plate wider or thicker. This may take up space on the circuit board and conflicts with the design goal of maximizing the number of electronic components on a circuit board.
Another characteristic of Parmerlee is that the heat transferred by the cold plate must travel across the circuit board for a long distance before it encounters the cooling liquid. In Parmerlee, the heat must travel through the cold plate to the edge of the circuit board to the parallel metallic sides to the cooling liquid. All cold plates have some thermal resistivity, which impedes the flow of heat and causes a temperature gradient, which decreases the efficiency of heat transfer.
While some of this disadvantage may be overcome by placing the electronic components that generate the most heat close to the parallel metallic sides, this conflicts with the design goal of placing electronic components according to functional rules, such as maximizing the speed at which the system operates.